The statements in this section merely provide background information related to the present disclosure. Accordingly, such statements are not intended to constitute an admission of prior art.
Known powertrain architectures include torque-generative devices, including internal combustion engines and electric machines, which transmit torque through a transmission device to an output member. One exemplary powertrain includes a two-mode, compound-split, electro-mechanical transmission which utilizes an input member for receiving motive torque from a prime mover power source, preferably an internal combustion engine, and an output member. The output member can be operatively connected to a driveline for a motor vehicle for transmitting tractive torque to the driveline. Electric machines, operative as motors or generators, generate a torque input to the transmission, independently of a torque input from the internal combustion engine. The electric machines may transform vehicle kinetic energy, transmitted through the vehicle driveline, to electrical energy that is storable in an electrical energy storage device. A control system monitors various inputs from the vehicle and the operator and provides operational control of the powertrain, including controlling transmission operating state and gear shifting, controlling the torque-generative devices, and regulating the electrical power interchange among the electrical energy storage device and the electric machines to manage outputs of the transmission, including torque and rotational speed. A hydraulic control system is known to provide pressurized hydraulic oil for a number of functions throughout the powertrain.
Operation of the above devices within a hybrid powertrain vehicle require management of numerous torque bearing shafts or devices representing connections to the above mentioned engine, electrical machines, and driveline. Input torque from the engine and input torque from the electric machine or electric machines can be applied individually or cooperatively to provide output torque. Various control schemes and operational connections between the various aforementioned components of the hybrid drive system are known, and the control system must be able to engage to and disengage the various components from the transmission in order to perform the functions of the hybrid powertrain system. Engagement and disengagement are known to be accomplished within the transmission by employing selectively operable clutches.
Clutches are devices well known in the art for engaging and disengaging shafts including the management of rotational velocity and torque transfer between the shafts. Application and release of clutches can be accomplished through hydraulic means and capable of a range of engagement states, from fully disengaged, to synchronized but not engaged, to engaged but with only minimal clamping force, to engaged with some maximum clamping force. The clamping force available to be applied to the clutch determines how much reactive torque the clutch can transmit before the clutch slips.
An hydraulic control system utilizes lines charged with hydraulic oil to selectively activate clutches within the transmission. Hydraulic switches or pressure control solenoids (PCS) can be used to selectively apply pressure within a hydraulic control system. Features within the PCS selectively channel or block hydraulic oil from passing therethrough depending upon the actuation state of the PCS. In a blocked state, a PCS is known to include an exhaust path, allowing any trapped hydraulic oil to escape, thereby de-energizing the connected hydraulic circuit in order to complete the actuation cycle. Modulation of the command pressure can enable the PCS to be linearly, variably actuated, including actuation controlling application of fill pressure to the clutch in order to achieve within the clutch some middle or transient state between full feed and exhaust states. In an exemplary transient state, the PCS, embodied as a variable bleed solenoid (VBS) can be operated with a portion of the hydraulic line pressure being used to maintain a desired clutch pressure, with a remainder of the hydraulic line pressure being bled back into a hydraulic return line.
Use of a PCS includes the use of comparatively heavy and expensive PCS hardware, and additionally, use of hydraulic bleed to maintain a desired or controlled pressure from the PCS returns some hydraulic line pressure that could otherwise be used in the hydraulic control system or could reduce the required output of the hydraulic pump. Use of a PCS can be desirable as it enables a staged fill event, wherein commands to the PCS can take a clutch through multiple stages culminating in the filling and engagement of the clutch. Such a staged process can be required to compensate for variables in the system, for example, hardware and a temperature of the hydraulic fluid, and to provide for smooth engagement of the clutch. According to one exemplary embodiment, it can be desirable to fill the clutch to a touching state, wherein a hydraulic cylinder of the clutch is filled with hydraulic fluid and just enough pressure is applied to the clutch plates such that the plates are made to touch without any clamping force being applied to the clutch plates. According to one embodiment, such a touching state can be used to compress overall timing of a transmission shift, enabling the clutch fill to occur to the touching state prior to the clutch being synchronized or being brought to a same speed. However, if clutch control can effectively be achieved without use of a PCS, weight, cost, and efficiency of the hydraulic control system can be improved.
An hydraulically actuated clutch operates by receiving pressurized hydraulic oil into a clutch volume chamber. Hydraulic oil in this clutch volume chamber exerts pressure upon features within the volume chamber. A piston or similar structure is known to be utilized to transform this hydraulic pressure into an articulation, for example a translating motion or compressing force. In an exemplary hydraulically actuated clutch, pressurized hydraulic oil is used to fill a clutch volume chamber and thereby displace a clutch piston in order to selectively apply a compression force to the connective surfaces of the clutch. A restoring force, for example as provided by a return spring, is known to be used to counter the compressive force of the hydraulic oil. As described above, clutches are known to be engaged through a range of engagement states. An exemplary clutch with all hydraulic pressure removed can be in an unlocked state. An exemplary clutch with maximum hydraulic pressure can be in a locked state. An exemplary clutch wherein the plates of the clutch have been brought to the same speed but a clamping force has not yet been applied to the clutch plates can be in a synchronized state.
An engagement of a clutch, accomplished through a clutch fill event, is known to be accomplished as rapidly as possible, with some minimum hydraulic pressure being maintained to assure rapid flow of the hydraulic oil into the clutch volume. However, rapid engagement of a clutch can cause a perceptible bump in the vehicle and cause shortened life of the component involved. A shock absorbing device can be utilized to dampen the force of the rapid fill of the clutch volume chamber upon the clutch. For example, a wave plate including a spring feature can be used between the cylinder piston and the clutch to absorb rapid increases in hydraulic pressure. The touching state described above can be defined as the clutch filled with enough hydraulic oil to cause zero force contact of the wave plate.